This is an open-label study designed to describe the antiviral activity, immunologic effect and long-term safety of the following drug regimen: once-daily ddI with a twice-daily regimen of d4T, Nelfinavir and Ritonavir in HIV-infected subjects who are antiretroviral-treatment nanve or AZT experienced and infected for a minimum of 90 days with a plasma HIV RNA count >10,000 HIV RNA copies/mL within 28 days prior to starting study. In addition, the study will attempt to document antiviral activity in viral compartments, including peripheral blood, lymphatic (tonsillar) tissue, cerebrospinal fluid and genital secretions, and to kinetically model active-virus-producing and latent-virus-infected compartments in both peripheral blood and tonsillar tissue in aggressively treated HIV-infected subjects.